In recent years, a brushless motor is used for a battery cooling fan of a hybrid vehicle that has been commonly used, due to requests for improved lifetime, energy saving, and decreased noise of the battery cooling fan. For the cooling fan, air volume required by the fan and driving sound produced in the periphery of the fan are defined according to an air temperature and a temperature of a cooling target at each time. The cooling fan needs to be controlled so as to satisfy those requirements. It is known that variation in air volume depends on variation in rotary speed of a motor attached to an impeller (bladed wheel). It is known that driving sound and motor vibration depend on a rotational torque of the motor.
Incidentally, a three-phase brushless motor controlled with pulse width modulation (PWM) driving is controlled by a motor control apparatus in the following manner. First, the motor control apparatus detects a rotational position of a rotor from a rising edge and a falling edge of an output level of a Hall sensor. An actual rotary speed is calculated from a change amount of the rotational position per unit time. In the motor control apparatus, a switching pulse width of a metal oxide semiconductor field effect transistor (MOS-FET) installed in an inverter circuit that achieves the PWM driving is controlled according to the actual rotary speed thus calculated.
Therefore, due to variation in an attachment position of the Hall sensor and magnetization of an N-pole and an S-pole of a rotor magnet or other factors, the output level of the Hall sensor is not switched at a predetermined rotational position of the rotor. This disadvantageously shifts timing of the rising or falling edge. It is known that, for example, an energization phase or period from the inverter circuit to a motor coil is deviated by this shift. It is also known that torque ripple of the motor is increased, and vibration and noise are also increased.
As a countermeasure to such variation, a technology that detects an induced voltage to correct a relative positional shift of each Hall sensor has been known (for example, PTL 1).